Remove it with acid....start with 3:1 water/muriatic and then go 2:1 or more until it comes off. Rinse right away with clean water.
It will come back sooner or later....nothing you can do.
Thanks! Any idea what it is? It's above the waterline and almost never gets wet.
It's called efflorescence. moisture from behind the rock migrates onto the surface where it evaporates leaving behind the minerals it has carried with it.